On a typical cruise the cruise ships sell excursions the total price for all the days can be more then the cruise itself.

But some ports have swimable beaches or snorkeling etc which are either walking distance or easily accessible by an inexpensive local bus, while other ports it is near impossible to do anything without paying for an expensive excursion.

From your experience which ports in the Caribbean, Mexico, Central America, or South America did you find one could go to a beach, snorkeling etc without having to pay for an excursion and which ports is it necessary to pay for an excursion?

I most ports you'll have to pay to get to the beach . If your a first or novice cruiser I would probably use ship excursions. If you are confident there are a number of private companies that have the same excursions for less. Check the Meet on Board forums for your specific cruise , someone might have a private group forming a excursion.